This invention deals with polymers and polymer films as they pertain to packaging products in flexible packaging films where the package is of the type having an inner heat sealable layer and an outer abuse resistant layer of nylon. A variety of these films is available in the commercial market. Exemplary of such films are the structures. ##STR1##
The four layer structure has an inner heat sealable layer of Surlyn, an outer abuse resistant and heat resistant layer of nylon, and an internal gas barrier layer of ethylene vinyl alcohol copolymer (EVOH). The Plexar layer serves as an adhesive between the EVOH and Surlyn layers.
The three layer structure merely omits the EVOH gas barrier layer for uses that do not require high levels of gaseous barrier to protect the product.
Films of this nature are generally formed by multiple layer coextrusion wherein it is desirable to select polymer compositions for the individual layers such that their rheological properties are conducive to intimate and simultaneous parallel melt flows in the coextrusion process.
Films conventionally available have generally used nylon 6 for the nylon layer of the film. Nylon 6, however, presents processing problems in the formulation of the film because the high amount of heat required for processing nylon 6 into film contributes to degradation of other polymers in the film, such as EVOH. Thus special provisions are desired to provide for the co-existent extrusion conditions necessary to coextrude the film; and indeed certain process parameters are precluded by these special provisions.
This problem is addressed in Japanese No. 58 74,313, May 4, 1983, to Sumitomo Bakelite. A coextruded multiple layer film is disclosed therein of the nature ##STR2## where the nylon is specified as a copolymer which is 80% to 90% nylon 6 and 10% to 20% nylon 66. The Bakelite patent specifies that the melting point of the nylon polymer is close to that of the EVOH polymer. Since the melting point of the nylon polymer is close to the melting point of the EVOH polymer, the major problems of forming the film are thus resolved.
Films of the Bakelite type, however, are limited in their capacity for further heat processing. In heat sealing processes, and in thermoforming processes, the thermal operating ranges which yield acceptable results in the finished package are so narrow as to not be easily sustained over extended periods with commercially available heat controllers. For example, in the heat sealing process, while the nylon copolymer has a lower softening and extrusion processing temperature, as compared to nylon 6, which is conducive to coextrusion with EVOH, that same lower processing temperature creates problems in heat sealing the package closed. Heat flux that normally would be used with structures having nylon 6 as the outer layer is too high for the nylon copolymer, and the film may be burned through. Another problem associated with the quantity of heat flux required for the heat sealing is that the nylon copolymer may soften and adhere to the sealing bar, and cause jam-up of the packaging machinery. This problem may, at times, be corrected by reducing the amount of heat applied to the nylon layer by the sealing bars. In those cases where the problem may be corrected by reducing the heat applied, the film must spend more time in contact with the sealing bar, thus creating an economic penalty as by slower line speed.
Thus there exists a need for a polymer composition that would have the combination of properties of low temperature extrusion processability and high heat resistance for heat sealing operations.
It is an object of this invention to provide novel nylon compositions which are susceptible of low temperature extrusion and are more readily compatible with coextrusion with EVOH while, at the same time, being capable of withstanding the higher heat sealing temperatures normally associated with nylon 6.
It is a further object to provide single-layer films of nylon which have high heat stability like nylon 6 but have a lower temperature extrusion processability than nylon 6, while having a high capacity for accepting heat useful for fabrication processes.
It is a more specific object to provide a multiple layer film having nylon on one surface, and a heat seal layer on the other surface wherein the nylon has the high heat resistant characteristics of nylon 6 in combination with lower extrusion processing temperatures than nylon 6.